A critical evaluation of unexpected refractive outcomes following LASIK for a moderate to high myopic astigmatism: the relationship between the planned astigmatic treatment and effective astigmatic treatment

Session Details

Session Title: LASIK Outcomes I

Session Date/Time: Tuesday 16/09/2014 | 14:00-16:00

Paper Time: 14:00

Venue: Boulevard B

First Author: : S.Patel UK

Co Author(s): :    M. Bohač   N. Gabrić           

Abstract Details

Purpose:

When a -2.75DCx45 planned astigmatic treatment (PAT) is required and the post-op residual astigmatism (RA) is -0.50DCx45, the effective astigmatic treatment (EAT) is -2.25DCx45. That is, a simple difference between the planned treatment and residual astigmatic powers when there is no change in the axis. The calculation of the EAT is more complex when the residual astigmatism differs in both power and axis for example if the RA, in this case, was -0.75DCx180. We planned to develop a computational procedure to calculate the EAT in more complex cases and determine if there was any association between PAT and EAT in cases where the post-op residual astigmatism ≥0.50DC following unremarkable LASIK and healing for moderate to high myopic astigmatism using two separate platforms.

Setting:

University Eye Hospital “Svjetlost”, Zagreb, Croatia

Methods:

We calculated the planned astigmatic treatment (PAT) along an axis (a1) and the power of the residual astigmatism along the same axis (a2), the effective treatment (ET) along this axis =a1-a2. This was repeated for all axes in 5º steps resulting in 72 separate ET values covering 360º. A graphical plot of each ET value against meridian angle was sinusoidal and, the effective astigmatic treatment (EAT) was derived from the best fit sphero-cylindrical description of the sinusoidal construction. For the example under ‘purpose’ where the RA value was -0.75DCx180, the computed EAT value was -2.84DCx55. This approach was applied to PAT and post-op RA values obtained from individual cases treated with either Wavelight AllegrettoIQ400Hz (group I, n= 99) or Schwind Amaris750S (group II, n=79) for myopic astigmatism. The PAT and corresponding EAT values were analysed to determine if there were any statistical patterns that could be used to better understand the mechanisms that led to the unexpected surprising results.

Results:

Reporting the key findings, A) The effective astigmatic power (y) was significantly correlated with the planned astigmatic power (x) and sine of the planned axis (z). Multiple linear regression revealed, I) y = 0.829x-0.4034z- 0.324, (r=0.804, n=99, F=8.78, p=<0.001) II) y = 0.904x-0.0339z- 0.102, (r=0.874, n=79, F=122.9, p=<0.001) B) The sine of the effective astigmatic power axis (z1) was significantly correlated with the planned astigmatic power (x) and sine of the planned axis (z). Multiple linear regression revealed, I) z1 = 0.950z-0.0007x+ 0.0315, (r=0.949, n=99, F=441.7, p=<0.001) II) z1 = 0.865z+0.0143x+ 0.103, (r=0.910, n=79, F=132.9, p=<0.001)

Conclusions:

Linear regression predicts for Wavelight Allegretto platform, the effective astigmatic power is upto 20% less than expected when the PAT axes are between 45º and 135º. The effective astigmatic axis is closer to 2º when the planned axis is 180 º. And, for Schwind Amaris platform, the effective astigmatic power is upto 8% less than expected irrespective of planned axis. The effective astigmatic axis is closer to 40º & 42º when the planned axis is 45º and closer to 130º & 132º when the planned axis is 135 º. The calculation of the effective astigmatic treatment using the planned astigmatic treatment and the post-op residual astigmatism assists in better understanding the mechanism that can lead to unwanted surprise astigmatic corrections after LASIK and help perfect future applications by developing strategies preventing post-op residual astigmatism from becoming manifest.

Financial Interest:

NONE